1. Field of the Invention
This invention relates to pre-heating hydrocarbon feedstock materials prior to cracking the feedstock. More specifically, the invention relates to the vaporization of heavy oils prior to pyrolysis cracking in an apparatus and process of the Thermal Regenerative Cracking (TRC) type which is the subject of U.S. Pat. No. 4,061,562 to McKinney et al.
2. Description of the Prior Art
Production of olefins and, in particular, ethylene has been achieved to a great extent by thermally cracking hydrocarbon feedstock and rapidly quenching the cracked effluent to selectively produce the desirable olefins. Feedstock such as ethane, naphtha and similar materials have provided very desirable hydrocarbon feedstock for cracking to produce olefins.
Recently, the use of heavy oils, such as the vacuum tower distillates from crude oils and hydrocracker feedstocks have been used to produce olefins by thermal cracking or pyrolysis. Several problems attend the cracking of heavy oils to produce olefins. Regardless of the system used, whether it be pyrolysis performed in a tubular furnace or in a reactor tube by the use of heated particulate solids to provide the heat necessary for the endothermic reaction, the pretreatment of heavy oils has become difficult and expensive.
Therefore, it is desirable and, in fact, necessary as a practical matter to preheat the heavy oil or any liquid hydrocarbon feedstock to a reaction inlet temperature of 1,100.degree. F. Typically, steam is used as a diluent and to provide the heat necessary to vaporize the feedstock to achieve the proper inlet reaction temperature and phase condition. At present, preheating of the hydrocarbon feedstock is achieved by flashing the hydrocarbon feedstock with steam at a relatively high temperature; i.e. 1,400.degree. F. for heavy feed (1,053.degree. F. for typical light feeds). Conventionally, the hydrocarbon liquid is first heated in indirect heat exchange relationship to about 440.degree. to 500.degree. F. The heated liquid is then mixed with the superheated steam and flashed to 1,100.degree. F. from the vaporization mix temperature of 720.degree. F. This flashing is necessary since liquid hydrocarbon can not be totally vaporized by indirect exchange without incurring significant fouling of the equipment. In the case of heavy oils, the steam of hydrocarbon ratio is necessarily about 1:1. Because of the nature of the heavy oil feedstock, a relatively high temperature (1,400.degree. F.) and a high steam to hydrocarbon ratio of 1:1 is required to provide the necessary vaporization heat.
With particular reference to the above mentioned TRC apparatus and process, it is noted the TRC has maximum economic advantages when cracking heavy feedstocks (650.degree. F. end-boiling point) at low steam dilutions. In TRC systems having low residence times (i.e. between 0.05 to 2 seconds, at temperatures in the range of 1300.degree. to 2500.degree. F.), selectivity is favored by rapid and intimate mixing which is best accomplished with a vapor feed according to the subject invention, rather than a liquid feed.